1. Field of the Invention
The present invention relates to a digital-to-analog converter utilizing a pulse-width modulation (PWM) system.
2. Description of Related Art
In the past, in digital audio disk (DAD) players or the like, a digital-to-analog (D-A) converter utilizing a so-called PAM system, or a stepped-wave system, in which input digital data is converted to pulse-amplitude modulated (PAM) signals and finally to an analog signal, was widely used.
Recently, a D-A converter utilizing a so-called PWM system in which input digital data is converted to pulse-width modulated (PWM) signals and finally to an analog signal, has come into use.
In this case, in order to reduce quantization noise within the audio frequency domain and obtain necessary resolution, an over sampling method and a noise shaping method, disclosed, for example, in the gazette of Japanese Laid-open Patent Publication No. 61-177819, are used in combination and, thereby, 3-bit 7-value digital data D1 to D7 and PWM signals whose pulse widths are TO to 7TO corresponding to the values [1] to [7] of the digital data, as shown in FIG. 1, are generated.
The D-A converter utilizing the PAM system, from the principle upon which it is based, has good linearity and provides a converted analog output signal with small distortion. However, it requires high precision current adders functioning accurately corresponding to the weights accorded the bits of the input digital data and an electronic switch operating at accurate timing. Hence, there is the difficulty that the circuit scale becomes large when it is attempted to increase its resolution, and, in addition, the overall circuit must be structured with high precision.
Although the conventional D-A converter has the advantage that its circuit configuration is simple, the disadvantage is that it generates even harmonic distortion which degrades the analog signal.
Such distortion is remarkable when the signal frequency is high or the pulse rate is low. For example, as shown in FIG. 2, with respect to a fundamental wave at 10 kHz, relatively large distortion appears especially in the second harmonic domain.
In order to overcome the above described difficulties, the present applicant has already proposed a "digital-to-analog converter" utilizing a PWM system, in which a differential component between a pulse-width modulated waveform corresponding to the input digital data and a "complementary" pulse-width modulated waveform corresponding to the 2s-complement data of the input digital data is output by a differential amplifier means, and the high-frequency component of the differential output waveform is eliminated to thereby obtain the analog output signal with its harmonic distortion reduced (U.S. patent application Ser. No. 479,163, filed Feb. 13, 1990, now U.S. Pat. No. 5,021,788).
The proposed digital-to-analog converter, however, requires two PWM circuits because it generates a pair of PWM signals complementary to each other and, hence, its structure becomes rather complex.